We will investigate the inadequate neutrophil supply in neonates and its relationship to mortality in neonatal sepsis. Modern therapeutic measures have had little impact on the outcome of neonatal sepsis, which accounts for 25 percent of the deaths during the first days of life, and appears to occur because the host defenses of neonates are not completely developed. Since hematologic studies of septic neonates exhibit neutropenia and exhaustion of the neutrophil supply, and since neutrophil transfusion appears to salvage at least some septic infants, we propose that the mechanisms of neutrophil production in neonates are incapable of providing a neutrophil supply large enough to resist infection. Therefore, we propose to investigate 1) the kinetics features of neonatal neutrophil production which contribute to inadequate neutrophil supply, 2) the ability of neonates to deliver neutrophils to inflammatory sites, 3) the ability of bacteria-specific antibody to protect septic neonates either before or after neutrophil supply exhaustion occurs, and 4) the effect of antibody and neutrophil transfusion together to salvage infants animals with otherwise lethal infection. We postulate that the information gained may be useful in devising effective treatment programs for human neonatal sepsis.